ABSTRACT Throughout the world Chlamydia trachomatis (Ct) is the most common sexually transmitted bacterial pathogen. In countries with poor sanitary conditions ocular infections with Ct can result in trachoma, the most frequent cause of preventable blindness worldwide. Based on protection and serological studies in mice Ct is classified into 15 major serovars of which eight account for most of the sexually transmitted infections (STI). In men and women, the majority of genital Ct infections are asymptomatic and therefore not treated. Even in symptomatic cases unless therapy is implemented in a timely and correct manner long-term sequelae including pelvic inflammatory disease (PID), chronic abdominal pain, ectopic pregnancy and infertility can occur. Thus, vaccination is the best approach to control chlamydial infections. In this Project we want to test the hypothesis that a subunit vaccine formulated with Ct antigens elicits robust immune responses and induces cross-serovar protection in female mice against a genital challenge. To accomplish this goal we are going to test, in C3H/HeN female mice, vaccines formulated with the Ct major outer membrane protein (MOMP) and the polymorphic membrane proteins (Ppms). MOMP, MOMP peptides, MOMP DNA and RNA, from Ct senior serovars, will be formulated and delivered using Nano-Scaffold-Based Nano Lipoprotein Particles (NLP). Immunization with native MOMP will help guide the production of these vaccine constructs. To enhance protection in the genital tract, mice will be vaccinated by mucosal and systemic routes using adjuvants. Following vaccination, mucosal and systemic cellular and humoral immune responses will be determined and correlated with protection. Vaccinated animals will be challenged transcervically with the eight most clinically relevant Ct serovars. Protection against infection will be assessed by determining the number of mice with positive vaginal cultures, the total number of positive vaginal cultures, the length of time mice shed and the severity of vaginal shedding. To determine protection against long-term sequelae vaccinated and challenged female mice will subsequently be caged with proven male breeder mice and fertility and upper genital pathology, including hydrosalpinx formation, will be evaluated. If we cannot obtain a robust, broad cross-serovar protection by vaccinating only with MOMP, we will test a polyvalent vaccine with MOMP in combination with Ppm C, G or H using NLP. To ascertain the neutral, additive, synergistic, or antagonistic effects of each antigen, following vaccination, immune responses to each antigen will be characterized and the data will be correlated with protection. Our goal is to engineer a safe Nano-Scaffold-Based subunit vaccine that protects mice against a genital challenge with the Ct serovars that infect the human genital tract. At the end of this project protective vaccine constructs will be ready for scale-up production and human implementation. An efficacious vaccine against Ct will have a broad worldwide health and socioeconomic impact.